1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Various measuring systems and apparatuses have been hitherto suggested in order to know the concentration of a desired gas component in a measurement gas.
For example, those known as the method for measuring NOx in a measurement gas such as combustion gas include a technique in which the NOx-reducing ability of Rh is utilized while using a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia to measure an electromotive force generated between the both electrodes.
The sensor as described above suffers the following problem. That is, the electromotive force is greatly changed depending on the change in concentration of oxygen contained in a combustion gas as a measurement gas. Moreover, the change in electromotive force is small with respect to the change in concentration of NOx. For this reason, the conventional sensor tends to suffer influence of noise. Further, in order to bring out the NOx-reducing ability, it is indispensable to use a reducing gas such as CO. For this reason, the amount of produced CO is generally smaller than the amount of produced NOx under a lean fuel combustion condition in which a large amount of NOx is produced. Therefore, the conventional sensor has a drawback in that it is impossible to perform measurement for a combustion gas produced under such a combustion condition.
A system has been disclosed, for example, in Japanese Laid-Open Patent Publication Nos. 63-38154 and 64-39545, in which a pair of electrochemical pumping cell and sensor cell comprising Pt electrode and an oxygen ion-conductive solid electrolyte are combined with another pair of electrochemical pumping cell and sensor cell comprising Rh electrode and an oxygen ion-conductive solid electrolyte to measure NOx in accordance with a difference between respective pumping current values.
Further, for example, Japanese Laid-Open Patent Publication Nos. 1-277751 and 2-1543 disclose the following method. That is, two pairs of electrochemical pumping cells and sensor cells are prepared. The limiting pumping current is measured at a partial pressure of oxygen at which NOx is not reduced, by using a sensor comprising one of the pairs of pumping cells and sensor cells, while the limiting pumping current is measured at a partial pressure of oxygen at which NOx is reduced, by using a sensor comprising the other pair of pumping cell and sensor cell so that the difference between the limiting pumping currents is determined. Besides, the difference in limiting current is measured by using a sensor comprising a pair of pumping cell and sensor cell, while switching the partial pressure of oxygen in a measurement gas between a partial pressure of oxygen at which NOx is reduced and a partial pressure of oxygen at which NOx is not reduced.